Method of producing benzene polycarboxylic acid



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METHOD OF PRODUCING BENZENE POLY- C r OXYLIC ACID Anton Benning, WernerEmte, and Otto Grossldnsliy, Dortmund-Eving, and Heinrich Fruhbuss,Munich, Germany, assignors to Bergwerksverband G.rn.h.H., Essen, GermanyNo Drawing. Filed June 14, 1955, Self. No. 515,561 Claims priority,application Germany Feb. 25, 1954 Claims. (Cl. 260-524) derivativescontaining both chloromethyl and alkyl substituents in the benzene ringby means of a cheap oxidizing agent such as nitric acid does not resultin completely oxidized products, e.g. in benzene polycarboxylic acidswherein carboxyl groups are the only substituents in the benzene ring.Attempts to carry out such oxida tion with nitric acid at atmosphericpressure did not result in completely oxidized products, while attemptsto carry out the oxidation with nitric acid at superatmospheric pressureresulted in poor yields of acids contaminated to a great extent withchlorinated and nitrated acids which are extremely difiicult to separatefrom the desired product.

It is accordingly a primary object of the present invention to provide amethod of oxidizing benzene derivatives containing both chloromethyl andalkyl groups as substituents in the benzene ring by means of nitric acidin a simple and inexpensive manner whereby high yields of easilyisolatable and uncontaminated benzene carboxylic acids are obtainedwithout the formation of considerable amounts of byproducts.

It is another object of the present invention to provide a method ofoxidizing chloromethylated alkyl benzenes with nitric acid whereby thenitric acid utilized is regenerated and returned to the oxidizingprocess.

It is still another object of the present invention to provide a methodof producing terephhalic acid in good yield by the oxidation ofp-chloromethyl toluene.

It is yet another object of the present invention to provide a method ofproducing high grade pyromellitic and trimellitic acid from cheap crudexylene.

It is a further object of the present invention to provide a method ofproducting mixtures of trimellitic and pyromellitic acid.

It is still a further object of the present invention to provide amethod of producing benzene carboxylic acids containing any number ofcarboxyl groups and wherein the carboxyl groups may be placed in themolecule in any desired position.

Other objects and advantages of the present invention will be apparentfrom a further reading of the specification and of the appended claims.

With the above objects in view, the present invention mainly consists ina method of producing benzene polycarboxylic acids, comprising the stepsof heating a benzene derivative consisting of a benzene ring substituted2,9524 Patented Dec. 27, 1560 with at least one alkyl group and with atleast one chloromethyl group with nitric acid at substantiallyatmospheric pressure at a temperature sufiiciently high to oxidize thechloromethyl group until a substantially chlorine-free reaction productis formed, thereby converting the chloromethyl group to a carboxyl groupwithout substantially affecting the alkyl group and thus formingessentially an alkyl benzene carboxylic acid, and heating thechlorine-free reaction product with nitric acid under superatmosphericpressure at a temperature sutficiently high to oxidize the alkyl group,thereby converting the alkyl group to a carboxyl group and thusobtaining a benzene carboxylic acid consisting of a benzene ringsubstituted only with carboxyl groups.

The starting materials for the method of the present invention arechloromethylated alkyl benzene derivatives such as toluene, Xylenes,ethylbenzene, mesitylene, durol, and the like or mixtures thereof. Bythe chloromethylation of these alkyl benzene derivatives, which per seis known, one or two hydrogen atoms of the benzene nucleus are replacedby chloromethyl groups (-CH Cl), thus producing benzene derivativescontaining one or two chloromethyl groups in addition to the alkyl groupor groups, e.g. CH C H etc., already present in the original alkylbenzene derivative.

In practice the chloromethylation is carried out by treating alkylbenzenes with chloromethyl ether or hydrogen chloride and formaldehyde(or its polymerides), whereby, depending on the reaction conditions, oneor two chloromethyl groups are incorporated into the benzene ring. Thus,for example, toluene can be converted into a derivative represented bythe formula C H .CH (CH Cl) xylene is analogously converted into 0rC6H2(CH3)2 (CH2Cl)2, etc. Subsequently, the chloromethylated benzenehomologues of the kind aforesaid are subjected to the twostage oxidationtreatment of the present invention, the first stage being carried out atsubstantially atmospheric pressure to convert the chloromethyl group orgroups to a carboxyl group or groups, and the second stage being carriedout at superatmospheric pressure to convert the non-converted alkylgroup or groups to a carboxyl group or groups. Thus, the number ofcarboxyl groups in the final benzene carboxylic acid is equal to thenumber of alkyl groups plus the number of chloromethyl groups present inthe original benzene derivative.

According to a preferred embodiment of the present inventionp-chloromethyltoluene is oxidized in the above described two-stageoxidation method to produce substantially pure terephthalic acid.

According to another embodiment of the present invention the totalnumber of substituents convertible to carboxyl groups by oxidation, i.e.the total number of alkyl and chloromethyl groups is three, whereby ahenzene tricarboxylic acid, such as trimellitic acid, is formed. Thetotal number of three oxidizable groups may be reached by onechloromethyl group and two alkyl groups, or two chloromethyl groups andone alkyl group.

In accordance with the present invention the total number of alkyl andchloromethyl groups may be four, five or six whereby the resultingbenzene carboxylic acid will be respectively pyromellitic acid, benzenepentacarboxylic acid or mellitic acid.

Preferably the alkyl groups of the alkyl benzene are methyl groups, e.g.as in toluene, xylenes, mesitylene, durol and the like.

It has been found according to the present invention that only by theabove described two-stage oxidizing treatment is it possible to obtainpure benzene polycarboxylic acids in excellent yield, with all of thesubstituents having been oxidized to carboxyl groups. In view of theuseless attempts made prior to the present invention to achieve asatisfactory and complete oxidation at elevated pressure, the new methodof the present invention is quite surprising as to the excellent yieldsand simplicity of proceeding. Prior to the present invention completeoxidation could only be achieved by means of expensive potassiumpermanganate or expensive roundabout procedures. The process of thepresent invention allows for the production of all the benzenecarboxylic acids, including the trivalent acids and their derivatives upto mellitic acid, starting almost and in the most interesting cases fromreadily obtainable benzene derivatives, especially toluene, and pure andcrude xylenes. Moreover, the process is extra-ordinarily simple in itsperformance, as will be seen from the following description.

The most suitable temperature for the heating during the first stage isthe boiling point of the nitric acid, about 105 C. The first stageshould be carried out at about atmospheric pressure although a slightexcess pressure may result in the speeding of the reaction in the firststage without adversely affecting the entire reaction.

During the second stage the superatmospheric pressure should be at least3 atmospheres, preferably between 3 and 30 atmospheres and mostpreferably between 5 and 20 atmospheres. A pressure of about 5atmospheres gives highly satisfactory results although the reactionproceeds faster at a pressure of about 15 atmospheres and at atemperature of about 150200 C.

The method can be carried out in practice in a single reactor, so thatisolation of the intermediate product is unnecessary and the operatingconditions only need be intensified as soon as all the chlorine has beenseparated from the starting material. This clearly necessitates areactor which is resistant to nitric acid and hydrochloric acid, e.g.such as one of nickel alloyed with 16-18% of molybdenum, l5.5-l7.5% ofchromium, 4.5-7% of iron, 3.75-4.75% of tungsten and small amounts ofsilicon or manganese.

Alternatively, the difiicultly soluble crude acid of the first reactionstage can be separated and further oxidized in a second reactor.Isolation of the intermediate product and further oxidation in a secondreactor have various advantages. The intermediate product can be freedby washing with a suitable alkyl benzene, e.g. toluene, from the lasttraces of impurities, such as non-converted chloromethyl alkyl benzene,and so be in the best form for obtaining an end product which fulfillsthe highest requirements of purity. The oxidation in two separatereactors is also of advantage since the corrosive action of the reactionmixture is very diiferent in the two stages. It has been found that itis most advantageous to carry out the first stage in a reactor linedwith a material fully resistant to chlorine and chlorine compounds, e.g.in ceramic reactors. For carrying out the second oxidation stage,special autoclaves of alloy steel are suitable, such as those made ofnickel, chromium, molybdenum, tungsten, titanium or their alloys orcovered with such materials.

It is clearly not necessary in operating in vessels of differentmaterial to cool the reaction mixture of the first stage and separatethe product. If a saving in autoclaves is desired, in which the secondoxidation is to be carried out, the last traces of chlorine are removedfrom the intermediate product, this being achieved most safely byseparating and washing the crude acid with liquid alkyl benzenes, suchas toluene, xylene, and the like. Said alkyl benzenes are especiallysuitable for washing, offering the chance of being used further asstarting materials for the production of further amounts of benzenecarboxylic acids after enrichment with chlorine compounds according tothe process of the invention, needing no purification, e.g. bydistillation, for the purpose of re-employment as washing solvent.

The oxidation of chloromethyl derivatives can be carried out equallywell, as mentioned, in the same autoclave, merely by raising thepressure and temperature on completion of the first oxidation reaction.Such a process is particularly satisfactory since, in the firstoxidation stage, a stronger nitric acid than in the pressure stage leadsto the best results and the decrease in the ntiric acid concentrationafter the first oxidation stage leads to a suitable acid concentrationfor the second stage.

It has been found that the oxidation proceeds rapidly and Without thedisadvantage of obtaining final products contaminated by nitro compoundsor other by-products, by the utilization of a relatively highlyconcentrated nitric acid during the first stage of oxidation atsubstantially normal pressure and utilizing a relatively weaker nitricacid concentration during the second stage of oxidation undersuperatmospheric pressure.

The best results are obtained utilizing nitric acid of about 30-47%concentration in the first stage and about 20-45% concentration in thesecond stage. Preferably the concentration of nitric acid in the firststage is 38- 47% and most preferably, the concentration of nitric acidin the first stage is about 43-45%. Preferably, the concentration ofnitric acid in the second stage is between 3038% when operating with abenzene derivative containing at least three constituents oxidizable tocarboxyl groups and about 32-35% when starting with chloromethyltoluene.

Despite the difference between the most advantageous concentrations ofthe nitric acid in the first and second stages it is possible to subjectthe entire reaction mixture of the first oxidation stage withoutinterruption to the further oxidation carried out under pressure. It hasproved that in practice the concentration of the nitric acid in thefirst stage decreases accurately during the oxidation so as to attainthe most suitable concentration for the second stage. It goes withoutsaying that within the scope of the process of the invention it ispossible also to work with stronger or weaker nitric acids in bothstages. As a rule, however, the concentrations mentioned above give thebest results.

When working with a nitric acid concentration of about 38-47%, a ratherchlorine-free intermediate product is obtained in the first stage. Theabsence of chlorine is important for the quantitative course of thesubsequent oxidation as well as for saving the pressureapparatus. It isobvious that hydrogen chloride can seriously damage the apparatus,especially under elevated pressure. For carrying out the secondoperating stage, apparatus of special steel resistant to compression andnitric acid are suitable, such as those made of nickel, chromium,tungsten, titanium, molybdenum or their alloys, or, at least apparatusprovided with equipments and internal coatings consisting of suchmaterials. For the first oxidation stage and the chloromethylation it isrecommendable to operate in reactors made of material resistant tohydrochloric acid, e.g. in ceramic or enamelled reactors which may beless resistant to pressure, however.

The consumption of the acid is known to be of great importance to theeconomy of a nitric acid oxidation, and is caused by the reduction ofthe acid to elementary nitrogen. With nitrogen, nitrous gases areseparated out in such oxidation, which can be readily recovered bywashing. However, the nitric acid reduced to nitrogen is lost for anoxidation method.

It has now been found that in both oxidation stages the reduction ofnitric acid to nitrogen can be substantially completely supressedwithout affecting or slowing the reaction. For this purpose, oxygen orair is passed through the reaction mixture during the reaction. Theexpected frequent explosions do not occur. The gas is introduced infinely-divided form from below into the reactor, so that it risesthrough the reaction mixture in the form of small bubbles. The nitricacid in this mode of operation is reduced substantially only to nitrogendioxide, which can be extracted and returned to the reactor if required.In this way, the method proceeds without significant losses of nitricacid, so that the oxygen required for the oxidation is suppliedsubstantially exclusively by the gases introduced. The reaction proceedsfaster and with the best yields, since too great a lowering of thenitric acid concentration during the reaction is avoided. If operatinginstead without simulaneous introduction of oxygen, the second oxidationstage must be carried out with higher pressures and temperatures inorder to achieve approximately equivalent results.

In the oxidation of the chloromethylated alkyl benzene in the firststage, not only is nitric oxide evolved from the reaction mixture, butalso hydrogen chloride, which can likewise be recovered. For thispurpose, the gas is passed for instance through concentrated sulfuricacid. Nitrosylsulfuric acid is formed from which nitric acid can bereadily obtained, while the hydrogen chloride gas evolves from thesulfuric acid and can be used again for preparing chloromethylatedderivatives.

A vertical heatable reaction tube is particularly suitable forcontinuous oxidation, in which the reaction components are introducedfrom below and the gas through a frit or the like. The reaction mixturerises in the tube according to the amount of fresh starting materialsupplied, without too great an intennixing of the reaction mixtureoccurring along the tube. The supply of new starting material iscontrolled so that reacted mixture always extends to the upper end ofthe tube. The mixture can be transferred to a container in which thedesired acid separates. From the container, which can be cooled ifrequired, the mother liquor flows over into a cooling tube. Togetherwith the mother liquor, the nitrous gases flow downwards through thetube, wherein all the nitrous gases are surprisingly taken up again bythe mother liquor, so that at the foot of the cooling tube nitric acidis obtained which is fully utilizable for the method and can be returnedto the reactor if necessary. That all the nitrous gases are taken up bythe nitric acid is shown since in the waste gas which is blown from thecontainer through a valve from the enclosed system, substantially nonitrous gases remain.

As aforesaid, the oxidation can be carried out without interruptionmerely by changing the operating conditions in a single apparatus. This,however, necessitates an apparatus resistant to hydrochloric acid and tonitric acid or mixtures thereof, respectively. For the reasons pointedout already it is preferable, however, to change the apparatus. In thiscase it may be very advantageous to separate and to wash theintermediate oxidation product. This measure is especially recommendablewhen obtaining intermediate products difficultly soluble in nitric acid,since the separating does not cause any difficulty, resulting moresafely in chlorine-free intermediate products.

The chloromethylation preceeding the oxidation of the benzenederivatives suitable as starting materials for the however, to producepreferably one of the two chloro-' methyl compounds. In general, thechlorornethylation' of e.g. m-xylene results in4-chloromethyl-l,3-dimethyl-' benzene and4,6-bis-chlorornethyl-1,3-dimethylbenzene. This mixture is obtained forexample by treating m-xylene at about -70 C. with four to five times itsweight of hydrochloric acid and about the same amount by Weight offormaldehyde with introduction of hydrogen chloride and vigorouslyagitating. When formaldehyde is present in larger excess, thebis-compound is preferably obtained. The non-reacted formaldehyde,paraformaldehyde etc. is not lost but is completely consumed by repeatedemployment of the reaction solutions. If bis-compounds are to beproduced exclusively, the mono-compounds are separated and subjected toanother chloromethylation. In this way trimellitic acid or otherbenzene-tricarboxylic acids are obtained from toluene via thebischlo-romethyl-toluene as intermediate products, and pyromellitic acidis obtained from xylene via the bis-intermediate products mentionedabove.

A special advantage of the process of the invention is the fact that theprocessing of practically all the xylene isomers leads exclusively totrimellitic and/ or pyromellitic acid. Consequently, the new processoffers the chance of producing these high-grade acids from the so-calledcrude xylenes. According to their origin these crude xylenes represent avarying mixture of 0-, mand p-xylene. The crude xylene of coke ovenplants contains for example about of m-xylene, 20% of p-xylene, 5% ofo-xylene, and, in addition, about 5% of ethylbenzene. The crude xylenesof the petroleum industry consist of isomers, too. Separating the singleisomers is diflicult due to their boiling points being closely together.

Consequently, the single pure isomers represent comparatively expensivestarting materials; on the other hand the crude xylenes are very cheapproducts. Thus the considerable importance of the process starting fromany arbitrary crude xylene and resulting in pure trior pyromelliticacid, is self-explanatory.

It may be additionally mentioned that by the process of the inventioninferior amounts of hemimellitic and1,2,3,4-benzene-tetra-carboxylic-acid are formed. Both acids can bereadily separated from trimellitic and pyromellitic acid and partiallydo not crystallize at all out of the oxidation solution together withthe latter acids. As they result only in small amounts, as pointed out,a separation and isolation of these acids may be generally dispsenedwith.

To which extent almost exclusively triand pyromellitic acid result whenworking up pure and crude xylenes may become apparent by the followingequations surnming up the process of the chloromethylation of xylenes,the products obtained making obvious the result of the subsequentoxidation.

CHzCl C|1H2Cl -CH3 CH3 CH3 ClHzC- Ha CH CH3 CH3 (7H .(llHa CHgCl CHzCl lClHsC Ha CH; CH:

(2H3 (11H: (3H (|JH CH:

CH CH3 -CH3 CH3 CH;

CHiCl ClI-I2C CHgCl CHzCl 2C1 HzCl l oxidation l (30 OH ('30 OH O O OH-O O OH I H000- 30 OH 470 OH In addition to crude xylenes as startingmaterials for the production of benzene polycarboxylic acids, mainlytoluene, single xylene isomers, triand higher methylbenzenes and thelike are suitable, too. Derivatives of these compounds, such asnitro-derivatives and all the mixtures of the starting materialsconcerned are suitable also, certainly all the benzene derivatives ortheir mixtures which, after having been chloromethylated, containsubstantially at least 3 substituents oxidizable to carboxyl groups.

When starting from a crude xylene, the mixture mentioned already andconsisting of monoand bis-chloromethylxylene is obtained bychloromethylation, representing already a commercially valuable mixtureof acids consisting substantially of pyromellitic acid, in addition totrimellitic acid after having been oxidized according to the process ofthe invention. Instead of this, the mono-product (boiling range 105l25C.) can be separated by distillation from the mixture of the monoandbis-chloromethylated products, and the distillate and the residue can beoxidized separately, thus yielding trimellitic acid as well aspyromellitic acid either or both in isolated condition.

Furthermore, it has been found that in producing chloromethylatedproducts a separate and partial isolation of monoand bis-products isentirely possible. It has been found that when cooling the reactionsolution a part of the bis-compounds crystallizes out already and cantherefore be separated. By stronger cooling it is even possible toseparate the greater part of the bis-compounds by simple means andsubsequently oxidize them separately. Consequently, the remainingsolution can be also removed separately or worked up as described above.Hereby the distillation residue is oxidized together also with the partseparated already, that is together with ali the bis-compounds. Thus,there is the chance to adjust the course of the process to therespective requirement of final products. The conditions illustrated forxylene are valid analogously for all other suitable starting materials.

When working up xylenes and crude xylenes, it has been found thatfurther possibilities of obtaining single benzene carboxylic acidsinstead of a mixture of acids are available. It has been found thatdecomposing the chloromethylated product into well defined fractions andsubjecting these fractions to separate oxidation is not absolutelynecessary. The acids resulting during the oxidation of the entirechloromethylated product, such as pyromellitic acid and trimellitic acidpossess quite different properties as to solubility and consequentlycrystallize out under completely different conditions. When cooling theoxidation Solution, pyromellitic acid precipitates quantitativelywithout simultaneous precipitation of thel,2,3,4-bcnzene-tetra-carboxylic-acid containing also four carboxylgroups, and can be readily separated from the oxidation solution.Trimellitic acid is highly soluble even in cold water, being notobtained therefore prior to evaporating the oxidation solution. Ifrequired, it can be readily freed from contaminations byl,2,3,4-benzene-tetra-carboxylic-acid and hemimellitic acid bysublimation or a similar method.

In carrying out the process in practice, it has surprisingly been foundwhen processing crude xylene, that terephthalic acid results asby-product in excellent purity and considerable amounts. It is known tobe difiicultly soluble even in hot nitric acid and consequently can bereadily separated from the hot oxidation solution of the pyromelliticacid crystallizing out not before being cooled. Instead of this, bothacids can be first separated together from the cold oxidation solutionand boiled with water whereby pyromellitic acid is dissolved, unliketerephtalic acid, which is then filtered off. The filtrate is cooled,the pyromellitic acid crystallizing out as the hydrate in purecondition.

On the other hand, when separating the mono-compound from thechloromethylated product of a crude xylene and separately oxidizing saidcompound, the terephthalic acid is obtained from the oxidation solutionin this very stage of the process in addition to trimellitic acid.

The terephthalic acid is formed from ethylbenzene contained in the crudexylene. Thus, the surprising purity of this acid is simultaneouslyexplained, since the ethyl group can be oxidized more readily than themethyl group. The formation of the less desired toluic acid resultingreadily as by-product when being produced from p-xylene, is preventedaccordingly.

Nevertheless it is surprising Without any doubt that the ethylbenzenecontained in commercial xylene is almost completely chloromethylated inpara-position under the usual conditions of chloromethylation,terephthalic acid being practically obtained according to the amounts ofethylbenzene contained in the crude xylene.

The following examples are given to further illustrate the process ofthe present invention, the scope of the invention not however beinglimited to the specific details of the examples. The benzenepolycarboxylic acids obtained in the examples are perfectly white sothat they can be used directly for preparing resins, lacquers orplasticizers.

Example I 100 parts of m-xylene, 200 parts of aqueous formaldehydesolution (30-40%) and 400 parts of concentrated hydrochloric acid areheated for six hours to 75% C. After adding another 200 parts offormaldehyde solution, the reaction mixture is heated to about 70 C.with introduction of hydrogen chloride for an additional ten hours.

The reaction solution is allowed to cool and the resulting oily crystallayer is separated. The crystals contained therein are filtered. partsof 4-6-bis-chloromethyl-1.3-dimethyl-benzene (melting point 98 C.) are,

obtained. The liquid reaction products are washed with water. The yieldis 95 parts of chloromethylated products (dry weight).

The crystalline bis-chloromethyl compound (75 parts) is heated toboiling with 750 parts of 40% nitric acid in a vessel having a ceramiccoating, for two hours with introduction of oxygen. The resultinginsoluble acid is filtered off and the filtrate-4f necessary afterreplenishing the nitric acid-is used for oxidizing otherchloromethylated products. The carboxylic acid obtained is oxidized with500 parts of 30% nitric acid for one hour at 170 C. under atmospherespressure while simultaneously introducing air. 75 parts of pyromelliticacid are obtained which crystallize out of the oxidation solution uponcooling.

95 parts of the remaining chloromethylated product are heated for twohours at 105-ll0 C. with 900 parts of 45% nitric acid while introducingair. The hot emulsion is pumped into a stainless steel reactor andoxidized for one hour at 175 C. under 12 atmospheres pressure, air beingpassed through simultaneously. Upon cooling the oxidation solution, 50parts of pyromellitic acid crystallize out yielding an acid of the acidnumber 880 and the melting point 270275 C. upon being recrystallizedfrom water. The nitrous acid reaction solution is evaporated underreduced pressure until the crystallization starts, and the nitric acidis recovered. 40 parts of trimellitic acid are obtained. I

Consequently the yield according to this manner of operating amounts to125 parts of pyromellitic acid and 40 parts of trimellitic acid.

For comparison, the aforesaid 75 parts of crystallinebis-chloromethylated product are directly treated at 170 C. and 10atmospheres of pressure with 750 parts of 32% nitric acid. A crude acidis obtained containing 28 parts of di-methyl-benezene-di-carboxylic acidplus 43 parts of pyromellitic acid.

Example II 100 parts of m-xylene are chloromethylated as described inExample I, but after the separation of the crystals (75 parts) theliquid fraction of the chloromethylated products is distilled off. At apressure of 14 mm. Hg and a temperature of 100110 C. 45 parts of4-chloromethyl-1,S-dimethylbenzene are obtained as distillate, while thedistillation residue becomes crystalline upon being cooled, consistingof 50 parts of bischloromethylxylene.

The aforesaid 45 parts of mono-chloromethylxylene are oxidized with 400parts of nitric acid (density 1.27) for two hours at l05l10 C. withintroduction of air. The acids obtained are filtered off and oxidizedwith 300 parts of nitric acid (density 1.20) for one hour at 165 C. andunder 15 atmospheres of pressure, air being passed through the reactionmass. After evaporating the nitric acid, 45 parts of trimellitic acidare obtained.

If, however, the 45 parts of mono-chloromethylxylene are returned to thechloromethylation process, 58 parts of the bis-chloromethyl-compound areobtained there from. Consequently 75+50+58=183 parts are available,yielding, when oxidized in the manner described with reference to themono-product and carried out in the same proportion of ingredients, 185parts of pyromellitic acid.

Thus, Example 11 shows the method of producing from 100 parts ofm-xylene either 185 parts of pyromellitic acid or 45 parts oftrimellitic acid and 130 parts of pyromellitic acid, said 130 parts ofpyromellitic acid resulting by oxidizing the aforesaid (75+50) parts ofthe bis-chloromethyl-compound.

Example II.

100 parts of crude xylene are heated for ten hours to 10 tion (30-40%)and 500 parts of concentrated hydro chloric acid, introducing hydrogenchloride. The xylene is not'added until the aqueousformaldehyde-hydrochloric-acid-solution is saturated with hydrogen chloride. After the reaction is completed the organic phase is separatedstill in hot condition, so that the bis-chloro-' methyl-xylenes cannotcrystallize out. For this reason the washing with water is done also inhot condition. 160 parts of chloromethylated products are obtained.

These products are heated to boiling for two hours with 1500 parts ofnitric acid (density 1.26) with introduction of air. The mixture ofacids obtained is filtered off and oxidized with 1000 parts of 35%nitric acid for one hour at 175 C. under 10 atmoshperes of pressure, airbeing passed in, too. After evaporating the nitrous acid reactionsolution, 160 parts of a mixture of acids are obtained consisting ofpyromellitic, trimellitic, terephthalic acid and small amounts of1,2,3,4-benzene-tetra-- carboxylic-acid, hemimellitic and phthalic acid.

However, when directly filtering oit the insoluble substances (still inhot condition) from the resulting nitric acid oxidation solution, 9parts of terephthalic acid are obtained. After being washed with hotwater, the acid number of the terephthalic acid is 674. When cooling thehot terephthalic acid filtrate (oxidation solution and Wash-water), 109parts of pyromellitic acid crystallize out, which are recrystallized inWater, the acid number being 880 and the melting point 269273 C. afterdrying. Then the nitrous reaction solution is evaporated and theremaining acids are recovered. 35 parts of a mixture of acids areobtained consisting substantially of trimellitic acid.

Example IV parts of crude xylene are chloromethylated as described inExample III, parts of chloromethylated products being obtained.

160 parts of the chloromethylated product are heated for two hours atl051l0 C. with 1500 parts of nitric acid (density 1.28) withintroduction of air. The mixture of acids obtained is filtered off andoxidized with 1000 parts of 33% nitric acid for one hour at C. under 15atmospheres of pressure, air being passed through the reaction mass. Theoxidation being complate, the solution is allowed to cool and theinsoluble substance is filtered 01f. The filtrate is evaporated, thusrecovering the nitric acid. The residue of the evaporation isrecrystallized in dilute hydrochloric acid, yielding 35 parts oftrimellitic acid. The filtered reaction solution can be used also tooxidize fresh substance, thus serving to enrich the trimellitic acid andother readily soluble carboxylic acids, such as1,2,3,4-benzene-tetracarboxylic-acid.

The part precipitated upon cooling the oxidation solution is boiled withwater and filtered in hot condition. 112 parts of pure pyromellitic acidcrystallize as a hydrate out or" the filtrate. The substance insolublein water (10 parts) consists of pure terephthalic acid.

Example V 100 parts of crude xylene are treated with formaldehyde andhydrochloric acid as indicated in Example I. The chloromethylationhaving been completed, the mixture is allowed to cool and the oilycrystal layer is separated from the aqueous reaction solution. Thecrystals are filtered off from the oily crystal layer and eventuallyrecrystallized in petroleum ether. The liquid chloromethylated productsare Washed with water and acid (density 1.28), air being passed throughthe reaction solution. The oxidation is carried out in a ceramic-coatedvessel. The hot oxidation solution is pumped into a stainless steelreactor and oxidized for one hour at 170 under atmospheres pressure,with introduction of air. The insoluble part is filtered off and washedwith hot water. 11 parts of terephthalic acid (acid number 674) areobtained.

Upon evaporating the filtrates, 39 parts of trimellitic acid crystallizeout. The mother liquids are collected and the remaining benzenecarboxylic acids are recovered from them.

Example VI 100 parts of 2,4-bis-chloromethyl-toluene are heated for twohours to boiling with 850 parts of nitric acid (density 1.28) withintroduction of air. The resulting crude acid is washed with toluene. Achlorine-free methylphthalic acid is obtained which is oxidized with 600parts of 32% nitric acid for one hour at 165l70 C. under 10 atmospherespressure, air being passed simultaneously through the oxidationsolution. After evaporating the nitric acid, 105 parts of trimelliticacid are obtained.

Example VII 100 parts of pseudocumene (1,2,4-trimethylbenzene) areheated for 3 hours to 70 C. with 200 parts of aqueous formaldehydesolution (3040%) and 500 parts of concentrated hydrochloric acid. Afteradding 100 parts of aqeuous formaldehyde solution, the mixture is heatedfor another 7 hours to 70 C. with introduction of hydrogen chloride. Thereaction solution is allowed to cool and the crystals are separated.Eventuallythey can be purified by re-crystallization in petroleum ether.140 parts of bis-chloromethyl-tri-methyl-benzene (melting point 84 C.)are obtained.

140 parts of the bis-chloromethyl-compound are heated to boiling for 2hours with 1200 parts of nitric acid (density 1.275) with introductionof air. The acid obtained is filtered off and oxidized for one hour at180 C. under atmospheres pressure with 900 parts of 35% nitric acid. Theoxidation solution is evaporated until crystallization starts, thus thenitric acid being recovered. 135 parts of benzene-penta-carboxylic acid(melting point 233-235 C.) are obtained.

Example VIII 100 parts of durene (or isodurene) are heated for 8 hoursto 75 C. with 300 parts of aqueous formaldehyde solution (30-40%) and400 parts of concentrated hydrochloric acid, additionally introducinghydrogen chloride. The oily reaction layer is separated and cooled. Thecrystals obtained are filtered off and the liquid chloromethyl fractionis returned to the chloromethylation. 90 parts of bis-chloromethyl-durol(melting point 193 C.) are obtained. When using isodurol, the meltingpoint of the bis-chloromethyl-compound is 107 C.

90 parts of bis-chloromethyl-tetra-methyl-benzene are oxidized withnitric acid in two stages as indicated in Example VII. After evaporatingthe nitric acid, 105 parts of mellitic acid are obtained which may, ifdesired, be re-crystallized from concentrated nitric acid.

Example IX 100 parts of p-chloromethyltoluene are heated to boiling fortwo hours at normal pressure in a vessel with a ceramic covering with850 parts of 32% nitric acid, with simultaneous introduction of air. 90parts of crude tolu'ic acid with 12% of chlorine are obtained. Its acidnumber is about 370. I

The crude acid is washed with toluene, whereby chlorine-free toluic acidis obtained having an acid number of 412. It is oxidized with 600 partsof 32% nitric acid for two hours at 160 C. and 5 atmospheres pres-Example X parts of pure 100 parts of p-chloromethyltoluene are heatedfor two hours at normal pressure at 105-110 C. with 900 parts of 45%nitric acid with the introduction of air. 97 parts of toluic acid areobtained in nitric acid solution, which is substantially free ofhydrochloric acid. The hot emulsion is pumped into a stainless steelreactor and oxidized for one hour at 170 C. and 10 atmospheres ofpressure, air being passed in simultaneously. Crude terephthalic acidwith an acid number of 660 is obtained. This acid gives parts ofterephthalic acid with an acid number of 673 after washing withmethanol.

By carrying out the oxidation without the addition of air, only 80 partsof terephthalic acid are obtained and 400 parts of nitric acid are lost,whereas only 20 parts of nitric acid are lost with simultaneousintroduction of air. The pressure must be raised to 15-20 atmospheresand the temperature to 180-200 C.

Example XI 100 parts of p-chloromethyltoluene are heated for two hoursat -110 C. with 900 parts of 45 nitric acid with the introduction ofoxygen. After separation and washing of the crude acid, 97 parts ofchlorine-free toluic acid are obtained. The toluic acid is admixed with32% nitric acid and pumped into a stainless steel reactor. Afteroxidizing for one hour at C. and 10 atmospheres, crude terephthalic acidwith an acid number of 665 is obtained. This acid yields 90 parts ofterephthalic acid with an acid number of 674 after washing withmethanol.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of producing benzene polycarboxylic acids, comprising thesteps of heating a benzene derivative consisting of a benzene ringsubstituted with at least one saturated alkyl group containing less than3 carbon atoms and with at least one chloromethyl group with nitric acidat substantially atmospheric pressure at a temperature sufiiciently highto oxidize said chloromethyl group until a substantially chlorine-freereaction product is formed, thereby converting said chloromethyl groupto a carboxyl group without substantially affecting said alkyl group andthus forming an alkyl benzene carboxylic acid; and heating saidchlorine-free reaction product with nitric acid under asuperatrnospheric pressure of 3-30 atmospheres at a temperaturesufiiciently high to oxidize said alkyl group, thereby converting saidalkyl group to a carboxyl group and thus obtaining a benzene carboxylicacid consisting of a benzene ring substituted only with carboxyl groups.p

2. A method according to claim 1 in which said benzene derivative isbis-chloromethyl xylene and said obtained benzene carboxylic acid ispyromellitic acid.

3. A method according to claim 1 in which said benzene derivative isp-chloromethyltoluene and said obtained benzene carboxylic acid isterephthalic acid.

4. A methodaccording to claim 1 in which said benzene derivative isbis-chloromethyl pseudocumene and said obtained benzene carboxylic acidis benzene-pentacarboxylic acid.

5. A method according to claim 1 in which said benzene derivative isbis-chloromethyl durene and said obtained benzene carboxylic acid ismellitic acid.

6. A method of producing benzene polycarboxylic acids, comprising thesteps of heating a benzene derivative consisting of a benzene ringsubstituted with at least one saturated alkyl group containing less than3 carbon atoms and at least one chloromethyl group, the total number ofsaid alkyl and chloromethyl groups being at least three, with nitricacid of 30-47% concentration at substantially atmospheric pressure at atemperature sufiieiently high to oxidize said chloromethyl group until asubstantially chlorine-free reaction product is formed, therebyconverting all chloromethyl groups to carboxyl groups withoutsubstantially affecting said alkyl groups and thus forming an alkylbenzene carboxylic acid; and heating said chlorine-free reaction productwith nitric acid of 20-45% concentration under a super-atmosphericpressure of 3-30 atmospheres at a temperature sufficient- 1y high tooxidize said alkyl groups, thereby converting said alkyl groups tocarboxyl groups and thus obtaining a benzene carboxylic acid consistingof a benzene ring substituted only with carboxyl groups.

7. A method of producing benzene polycarboxylic acids, comprising thesteps of heating a benzene derivative consisting of a benzene ringsubstituted with at least one saturated alkyl group containing less than3 carbon atoms and with at least one chloromethyl group with nitric acidat substantially atmospheric pressure at a temperature correspondingsubstantially to the boiling range of said nitric acid so as to oxidizesaid chloromethyl group until a substantially chlorine-free reactionproduct is formed, thereby converting said chloromethyl group to acarboxyl group without substantially affecting said alkyl group and thusforming an alkyl benzene carboxylic acid; and heating said chlorine-freereaction product with nitric acid under a superatmospheric pres sure of3-30 atmospheres at a temperature of about 150- 200 C. so as to oxidizesaid alkyl group, thereby converting said alkyl group to a carboxylgroup and thus obtaining a benzene carboxylic acid consisting of abenzene ring substituted only with carboxyl groups.

8. A method of producing benzene polycarboxylic acids, comprising thesteps of heating a benzene derivative consisting of a benzene ringsubstituted with at least one saturated alkyl group containing less than3 carbon atoms and with at least one chloromethyl group with nitric acidof between 30-47% concentration at substantially atmospheric pressure atabout the boiling point of the reaction mixture at atmospheric pressureso as to oxidize said chloromethyl group until a substantiallychlorine-free reaction product is formed, thereby converting saidchloromethyl group to a carboxyl group without substantially affectingsaid alkyl group and thus forming an alkyl benzene carboxylic acid; andheating said chlorine-free reaction product with nitric acid of between14 20-45% concentration under a superatmospheric pressure of 3-30atmospheres at a temperature of about ISO-200 C. so as to oxidize saidalkyl group, thereby converting said alkyl group to a carboxyl group andthus obtaining a benzene carboxylic acid consisting of a benzene ringsubstituted only with carboxyl groups.

9.-A method of producing benzen'e polycarboxylic acids, comprising thesteps of heating a benzene derivativeiconsisting of a benzene ringsubstituted with at least one saturated alkyl group containing less than3 carbon atoms and with at least one chloromethyl group with nitric acidat substantially atmospheric pressure at a temperature sufficiently highto oxidize said chloromethyl group while bubbling a gas containing freeoxygen through the reaction mixture until a substantially chlorine-freereaction product is formed, thereby converting said chloromethyl groupto a carboxyl group without substantially aifecting said alkyl group andthus forming an alkyl benzene carboxylic acid and evolving a gasincluding nitrogen oxide; and heating said chlorine-free reactionproduct with nitric acid under a superatmospheric pressure of 3-30atmospheres at a temperature sufiiciently high to oxidize said alkylgroup while bubbling a gas containing free oxygen through the reactionmixture, thereby converting said alkyl group to a carboxyl group andthus obtaining a benzene carboxylic acid consisting of a benzene ringsubstituted only with carboxyl groups and evolving a gas includingnitrogen oxide.

10. A method of producing benzene polycarboxylic acids, comprising thesteps of heating mono-chloromethyl xylene and nitric acid atsubstantially atmospheric pressure at a temperature sufiiciently high tooxidize the chloromethyl group of said mono-chloromethyl xylene to acarboxyl group until a substantially chlorine-free reaction product isformed, thereby converting said chloromethyl group to a carboxyl groupwithout substantially affecting the alkyl group of saidmono-chloromethyl xylene, thus forming dimethyl-benzene mono-carboxylicacid; and heating said dimethyl-benzene mono-carboxylic acid with nitricacid of 20-45% concentration under a superatmospheric pressure of 3-30atmospheres at a temperature sufficiently high to oxidize said methylgroups, thereby converting said methyl groups to carboxyl groups, thusotbaining trimellitic acid.

References Cited in the file of this patent UNITED STATES PATENTS1,332,028 Coblentz et al. Feb. 24, 1920 1,576,999 Seydel Mar. 16, 19262,636,899 Burrows et al. Apr. 28, 1953 2,740,811 Lotz Apr. 3, 1956FOREIGN PATENTS 494,439 Belgium Mar. 31, 1950 517,238 Belgium Feb. 14,1953 698,734 Great Britain Oct. 21, 1953

1. A METHOD OF PRODUCING BENZENE POLYCARBOXYLIC ACIDS, COMPRISING THESTEPS OF HEATING A BENZENE DERIVATIVE CONSISTING OF A BENZENE RINGSUBSTITUTED WITH AT LEAST ONE SATURATED ALKYL GROUP CONTAINING LESS THAN3 CARBON ATOMS AND WITH AT LEAST ONE CHLOROMETHYL GROUP WITH NITRIC ACIDAT SUBSTANTIALLY ATMOSPHERIC PRESSURE AT A TEMPERATURE SUFFICIENTLY HIGHTO OXIDIZE SAID CHLOROMETHYL GROUP UNTIL A SUBSTANTIALLY CHLORINE-FREEREACTION PRODUCT IS FORMED, THEREBY CONVERTING SAID CHLOROMETHYL GROUPTO A CARBOXYL GROUP WITHOUT SUBSTANTIALLY AFFECTING SAID ALKYL GROUP ANDTHUS FORMING AN ALKYL BENZENE CARBOLIC ACID, AND HEATING SAIDCHLORINE-FREE REACTION PRODUCT WITH NITRIC ACID UNDER A SUPERATOMSPHERICPRESSURE OF 3-30 ATMOSPHERES AT A TEMPERATURE SUFFICIENTLY HIGH TOOXIDIZE SAID ALKYL GROUP, THEREBY CONVERTING SAID ALKYL GROUP TO ACARBOXYL GROUP AND THUS OBTAINING A BENZENE CARBOXYLIC ACID CONSISTINGOF A BENZENE RING SUBSTITUTED ONLY WITH CARBOXYL GROUPS.